Many vehicles use inertial measurement units (IMU's) as sensors to predict the attitude of a vehicle in space. IMU's typically include one or more of a three-axis accelerometer and a three-axis gyroscope with sensor fusion algorithms. Generally, when the vehicle is stationary, the accelerometer is used to measure the direction of gravity. Generally, under dynamic conditions such as when the vehicle is moving, the gyroscope acts to measure the rate of change of the vehicle's attitude. These two measurements can be combined to provide a more accurate estimate of the vehicle's attitude.
Many IMU's utilize a vertical gyro sensor fusion algorithm. The vertical gyro assumes that most of the time the IMU does not accelerate in the horizontal plane. One of the detractors of this type of sensor fusion algorithm is that accelerations in the horizontal plane cause errors in the measured attitude of the sensor. As such, the lateral acceleration causes a roll measurement offset error. Other problems occur as a result of a vertical gyro sensor fusion algorithm when the vehicle starts moving. For example, determining when to switch between the three-axis accelerometer and the three-axis gyroscope and how to weight the measurements of the three-axis accelerometer and the three-axis gyroscope in the calculations. Typically, there is a time lag between the moment when the vehicle starts moving and the moment when the algorithm determines the vehicle is moving which results in an error that is inserted into the system. Moreover, the error increases or accumulates as the IMU sensor goes back and forth between the three-axis accelerometer and the three-axis gyroscope which adds more error into the system. Moreover, the start and stop accelerations cause a pitch measurement error.
Thus there is a need for improvement for using inertial measurement units.